The present invention relates to a method and apparatus for coating a moving web, such as a paper web moving at a web speed in excess of 400 m/min., whereby a coating agent is applied in excess on one side of the web. According to one method, the coating agent has a dryness content exceeding 60% and/or viscosity exceeding 1500 cp, and is applied upstream of a pressure nip for distributing and applying the coating agent, the web being fed between two members forming the pressure nip, one of said members constituting a movable support such as a rotating roller and the other constituting a blade having a bevel operating on the web.
Several different methods and apparatus operating with a flexible blade to coat one side of a paper web are already known. In one known proposal a paper web is passed downwardly, the web partially surrounding a support roller rotating at the same speed as the paper web. A flexible blade secured in a blade holder and forming an acute angle to the paper web seen in the direction of movement of the paper web presses the paper web against the support roller. The coating agent is supplied in the space between the blade and the paper web so that a dam of coating compound is formed. The quantity of coating compound applied on the paper web is determined amongst other things by the force with which the blade presses the web against the support roller. According to another embodiment, which has recently been preferred, the paper web is passed between two rollers, one of which constitutes a support roller rotating at the same speed as the paper web and the other constituting an application roller. The support roller is usually placed above the application roller. In this case the paper web is caused partially to surround the support roller. The application roller rotates in either the same or the opposite direction relative to the direction of movement of the paper web and is adjustably journalled so that a suitable space is obtained between it and the paper web. The coating material is supplied in a suitable manner to the application roller which in turn applies an excess of coating material on one side of the paper web. A flexible blade, secured in a suitable manner by a blade holder, presses the paper web against the support roller some distance downstream of the application point for the coating material. The excess coating material is thus scraped away and can be returned to the system in any suitable manner. At the same time uniform spreading and regulation of the desired application quantity of coating material on the web is obtained. The blade pressure can be set by suitable means. In a third known embodiment, similar to that just described, the movable application roller is replaced by a stationary means for applying coating material, said means being provided with an upper outlet slot through which the coating material is applied in excess on the paper web.
In all types of blade coating a spring-loaded blade presses against the paper web partly to scrape off excess coating material so that the requisite quantity of coating material remains on the paper, and partly to spread the material uniformly over the web. Due to the hydraulic forces caused by the relatively vigorous movement of the coating material during the coating process, a force is generated on the coating blade which counteracts the spring force of the blade itself. At high web speeds and/or high viscosities of the coating material, therefore, the spring force must be increased in order to compensate for the increased hydraulic pressure on the blade so that the desired quantity applied is obtained on the paper.
In all blade coating processes the blade edge angle, that is the angle between the blade edge and the web is acute. The reason for this is that a wedge action is then obtained so that particles, impurities and irregularities then pass more easily under the blade.
In all blade coating processes it is desirable to use coating materials having high dryness contents. This is partly because a coating material having high dryness content permits higher coating speeds without the requirement that the web subsequently pass through a drying section which is very large. Another advantage is that a smaller energy requirement is needed to dry off the water in the coating compound. Yet another advantage is that, for a certain coating speed, a smaller drying section can be used involving decreased investment and running costs. Thus, although it is always desirable to use coating materials with high dryness contents, there exists an upper practical limit for this. One reason is that a higher dryness content increases the risk of defects in the coated surface, primarily in the form of streaks. Another drawback is the increased risk of rupture when defects pass between blade and web, particularly when thin grades of paper are being coated.
The upper limit for the dryness content of the coating material is dependent on many factors, such as the type of coating material to be used for various reasons. The coating material generally consists of a dispersion of filler such as kaolin in water. The dispersion also contains binder and various additives. As mentioned earlier, streaks or stripes may easily occur in the coating layer if high dryness contents are used. Such streaks are of two distinct types: they may be mechanical or rheological. In both cases they are caused by solid particles or coagulated coating material which catches under the edge of the blade. The rheological stripes occur as follows: water in the coating material will be continuously transferred to a certain extent from the coating material to the base paper during its passage from the application point to the blade edge. Loss of water in the coating layer will thus cause an increase in the dryness content and therefore the viscosity of the coating material, resulting in coagulation due to the action of the cutting forces prevailing under the blade.
Although the risk of streaking can be reduced by a suitable choice of composition for the coating material, there is even here an upper practical limit. This limit varies considerably and is dependent on many factors. However, in general it may be stated that for conventional coating compositions the upper limit is around 60 - 65 for the dryness content, measured in percent by weight, although in certain cases even higher contents are possible. However, in such exceptional cases a special composition is necessary and the selection is thus limited, which is a drawback. As mentioned earlier, the manner in which the blade is arranged can also affect the risk of streaks. For instance, it is known that a small blade edge angle decreases the risk of streaks due to the wedge action. The various factors affecting the coating quantity applied in otherwise identical conditions are blade thickness, blade edge angle, pressure and the rheological properties of the coating material, etc.
Steel blades of spring steel having a thickness of between 0.25 mm and up to 0.70 mm are usually used for blade coating. The blade edge angle may be varied within wide limits. In most cases a blade edge angle of between 20 and 45 degrees is used but in many cases it has been found advantageous to use angles both smaller and larger. The blade is usually ground to a certain bevel before use and is mounted at a predetermined blade edge angle. Occasionally blades without a bevel are used. There are special occasions when extremely large quantities of coating material are required and the blade is then curved so much that the side of the blade lies parallel with the paper web. In this case there is no bevel at all. However, unless extremely large quantities are required a bevel is always used. In general, the more acute the blade edge angle the greater will be the quantity applied under otherwise identical conditions.
The spring force, in the following designated F, is selected taking into account the web speed and the viscosity of the coating material. The greater the speed and viscosity, the greater must be the spring force. The spring force is achieved by setting the blade holder in a suitable manner so that the blade is curved. The spring force is dependent on blade thickness, clamping length and elastic modulus. There are special arrangements in which the spring force can be increased by pressing on the blade between its attachment point and edge. The spring force is usually expressed in total force per unit width measured in the transverse direction of the web. The spring force, dependent on many factors, varies within wide limits. Within the speed range 300 - 700 m/min, using conventional coating compound with dryness contents around 60%, it is probably normally 0.5 - 1.5 kgf/cm width of the web. However, it has also been found that there is an upper practical limit for the spring force since the risk of web rupture increases if a high spring force is used, for instance, with a thick blade, possibly combined with short clamping length. When the blade is pressed away from the paper web during the starting instant, and the coating material has perhaps not completely covered the paper web, it is subjected to considerable strain since the hydraulic pressure is not fully developed and does not therefore completely counteract the spring force of the blade. In such cases the considerable strain on the paper during the starting instant may cause a web rupture.